Lithium Bis(trifluoromethanesulfonyl)imide Electrolytes Research Articles

Effect of Salt Concentration in Water‐In‐Salt Electrolyte on Supercapacitor Applications

AbstractElectrical double‐layer supercapacitors offer numerous advantages in the context of energy storage; however, their widespread use is hindered by the high unit energy cost and low specific energy. Recently, water‐in‐salt (WIS) electrolytes have garnered interest for use in energy storage devices. Nevertheless, their direct application in high‐power devices is limited due to their high viscosity. In this study, we investigate the WIS Lithium bis(trifluoromethanesulfonyl)Imide (LiTFSI) electrolyte, revealing a high specific capacitance despite its elevated viscosity and restricted ionic conductivity. Our approach involves nuclear magnetic resonance (NMR) analysis alongside electrochemical analyses, highlighting the pronounced advantage of the WIS LiTFSI electrolyte over the WIS LiCl electrolyte at the molecular level. The NMR analysis shows that the LiTFSI electrolyte ions preferentially reside within the activated carbon pore network in the absence of an applied potential, in contrast to LiCl where the ions are more evenly distributed between the in‐pore and ex‐pore environments. This difference may contribute to the difference in capacitance between the two electrolytes observed during electrochemical cycling.

High‐Entropy Cubic Perovskite Oxide‐Based Solid Electrolyte in Quasi‐Solid‐State Li–S Battery

The shuttle effect of polysulfides with its corresponding capacity fading and safety issues is the major barrier in the realization of lithium–sulfur battery (LSB) technology despite having high energy density and high specific capacity. Solid ceramic electrolytes using poly(ethylene oxide) (PEO) matrix has attracted much interest due to their superior safety compared to their liquid electrolyte counterparts. However, cubic perovskite electrolytes suffer from considerable interfacial challenges with lithium metal anodes. Besides, ceramic electrolytes are difficult to process further due to their inherent brittleness. Herein, a novel high‐entropy cubic perovskite (HE‐LLZO) prepared by the ball‐milling method is designed to which PEO matrix is added along with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) for use as a solid‐state electrolyte (PEO–HE–LLZO–LiTFSI) in LSB. This electrolyte gives good ionic conductivity and blocks lithium dendrite formation as well. A novel transition metal rare‐earth‐based high‐entropy oxide is incorporated into the sulfur cathode for use as a host material to ameliorate the redox kinetics. A quasi‐solid‐state LSB is designed with transition metal rare‐earth oxide carbon nanotubes incorporated into sulfur as composite cathode and PEO–HE–LLZO–LiTFSI electrolyte. This study highlights the viability of designing high‐entropy materials as a solid‐state electrolyte for next‐generation all‐solid‐state LSBs.

A poly(ether block amide) based solid polymer electrolyte for solid-state lithium metal batteries.

Solid-state lithium (Li) metal batteries (SSLMBs) with high-energy density and high-security are promising for energy storage application and electronic device development. However, Li dendrite generation is still one of the most important factors hindering the application of SSLMBs since interface contact degradation, dead Li accumulation, and continuous solid-electrolyte interphase (SEI) growth are always caused by Li dendrite growth, making the performances of SSLMBs deteriorate rapidly. In this study, a poly(ether block amide) (PEBA) based polymer electrolyte with lithium bis-(trifluoromethanesulfonyl)imide (LiTFSI) as the Li salt is developed. It is found that the PEBA 2533-20% LiTFSI electrolyte possesses an ion conductivity of 3.0×10-5 S cm-1 at 25 °C. Especially, the Li dendrite suppression ability of SEI is greatly enhanced since it provides abundant amide groups to activate TFSI- anions and further enriches lithium fluoride (LiF) content in the SEI layer, which endows the full-cell with enhanced cyclability. As a result, the fabricated solid-state Li/PEBA 2533-20% LiTFSI/LiFePO4 (areal capacity: 0.15 mAh cm-2) battery remains 94% of its maximum capacity (127.5 mAh g-1) at a rate of 0.5C and 60 °C after 200 cycles. In particular, the full cell can cycle for almost 1000 times without short circuit. Therefore, the PEBA based electrolyte could promote the LiF enriched SEI layer into a platform to suppress the growth of Li dendrite toward SSLMBs with a long-life span.

Effects of the Electrolyte Concentration on the Nature of the Solid Electrolyte Interphase of a Lithium Metal Electrode

The need of more powerful systems with higher energy density raises a lot of interest in lithium metal batteries (LMBs). As LMBs suffer from safety concerns due to the dendrite growth, several strategies have been studied to limit this growth. Using a highly concentrated electrolyte allows a homogeneous lithium plating that delays the formation of dendrites. Herein, different techniques are used in order to better understand the beneficial role of the salt concentration in the lithium plating/stripping. Operando Fourier transform infrared spectroscopy highlights the better reversibility of the Li+ solvation in the 5 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in 1,2‐dioxolane/1,3‐dimethoxyethane electrolyte in comparison with the 1 M electrolyte. This obviously leads to different electrolyte decompositions during the lithium plating/stripping and changes the nature of the electrode solid electrolyte interphase (SEI) depending on the salt concentration. Gas chromatography coupled with mass spectrometry as well as X‐ray photoelectron spectroscopy confirms that with the 5 M LiTFSI electrolyte the salt is preferentially reduced during the plating/stripping, leading to a more inorganic SEI on the lithium metal electrode.

Efficient and Stable Fiber Dye-Sensitized Solar Cells Based on Solid-State Li-TFSI Electrolytes with 4-Oxo-TEMPO Derivatives.

Fiber-shaped dye-sensitized solar cells (FDSSCs) with flexibility, weavablity, and wearability have attracted intense scientific interest and development in recent years due to their low cost, simple fabrication, and environmentally friendly operation. Since the Grätzel group used the organic radical 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) as the redox system in dye-sensitized solar cells (DSSCs) in 2008, TEMPO has been utilized as an electrolyte to further improve power conversion efficiency (PCE) of solar cells. Hence, the TEMPO with high catalyst oxidant characteristics was developed as a hybrid solid-state electrolyte having high conductivity and stability structure by being integrated with a lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) film for FDSSCs. The optimized 4-Oxo TEMPO (OX) based solid-state FDSSC (SS-FDSSC) showed the PCE of up to 6%, which was improved by 34.2% compared to that of the reference device with 4.47%. The OX-enhanced SS-FDSSCs reduced a series resistance (Rs) resulting in effective electron extraction with improved short-circuit current density (JSC), while increasing a shunt resistance (Rsh) to prevent the recombination of photo-excited electrons. The result is an improvement in a fill factor (FF) and consequently a higher value for the PCE.

Revealing the Nanostructures of Liquid Electrolytes By X-Ray Scattering

The solution structure of the liquid electrolyte directly affects its ion migration ability and interfacial insertion chemistry, which ultimately determines the performance of batteries. Herein, small-angle X-ray scattering-wide angle X-ray scattering (SAXS-WAXS) is employed to investigate the diffuse structure of carbonate electrolytes containing lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and lithium hexafluorophosphate (LiPF6), respectively. As the salt concentration increases, a upheave behavior is observed in both electrolyte systems at the high q part, which is caused by the cation-solvent interaction and charge ordering formation. Notably, at the low q part, we observe another gel polymer-like structure in the LiPF6 electrolyte system while the LiTFSI electrolytes do not have. It is believed that the anions cause this polymer-like structure, that is PF6- have strong hydrogen bonding ability than TFSI anions and, to some extent, cross-links solvent molecules. Raman, nuclear magnetic resonance (NMR), and molecular dynamics (MD) simulation further support the proposed structures. The structure-electrochemical performance relationship study suggests that LiPF6 electrolytes is more conducive to form stable SEI and reduce solvent-electrode side reactions. This work highlights the fundamental structure analysis in nanoscale, which will prompt better electrolytes in the future.

Intrinsic Ion Transport Properties of Block Copolymer Electrolytes.

Knowledge of intrinsic properties is of central importance for materials design and assessing suitability for specific applications. Self-assembling block copolymer electrolytes (BCEs) are of great interest for applications in solid-state energy storage devices. A fundamental understanding of ion transport properties, however, is hindered by the difficulty in deconvoluting extrinsic factors, such as defects, from intrinsic factors, such as the presence of interfaces between the domains. Here, we quantify the intrinsic ion transport properties of a model BCE system consisting of poly(styrene-block-ethylene oxide) (SEO) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt using a generalizable strategy of depositing thin films on interdigitated electrodes and self-assembling fully connected parallel lamellar structures throughout the films. Comparison between conductivity in homopolymer poly(ethylene oxide) (PEO)-LiTFSI electrolytes and the analogous conducting material in SEO over a range of salt concentrations (r, molar ratio of lithium ion to ethylene oxide repeat units) and temperatures reveals that between 20% and 50% of the PEO in SEO is inactive. Using mean-field theory calculations of the domain structure and monomer concentration profiles at domain interfaces-both of which vary substantially with salt concentration-the fraction of inactive PEO in the SEO, as derived from conductivity measurements, can be quantitatively reconciled with the fraction of PEO that is mixed with greater than a few volume percent of polystyrene. Despite the detrimental interfacial effects for ion transport in BCEs, the intrinsic conductivity of the SEO studied here (ca. 10-3 S/cm at 90 °C, r = 0.085) is an order of magnitude higher than reported values from bulk samples of similar molecular weight SEO (ca. 10-4 S/cm at 90 °C, r = 0.085). Overall, this work provides motivation and methods for pursuing improved BCE chemical design, interfacial engineering, and processing.

Simultaneous Improvement of Ionic Conductivity and Mechanical Strength in Block Copolymer Electrolytes with Double Conductive Nanophases.

The most daunting challenge of solid polymer electrolytes (SPEs) is the development of materials with simultaneously high ionic conductivity and mechanical strength. Herein, SPEs of lithium bis-(trifluoromethanesulfonyl)imide (LiTFSI)-doped poly(propylene monothiocarbonate)-b-poly(ethylene oxide) (PPMTC-b-PEO) block copolymers (BCPs) with both blocks associating with Li+ ions are prepared. It is found that the PPMTC-b-PEO/LiTFSI electrolytes with double conductive phases exhibit much higher ionic conductivity (2 × 10-4 S cm-1 at r.t.) than the BCP electrolytes with a single conductive phase. Concurrently, the storage moduli of PPMTCn -b-PEO44 /LiTFSI electrolytes are ≈1-4 orders of magnitude higher than that of the neat PEO/LiTFSI electrolytes. Therefore, simultaneous improvement of ionic conductivity and mechanical properties is achieved by construction of a microphase-separated and disordered structure with double conductive phases.

Highly Concentrated Aqueous Electrolyte With a Large Stable Potential Window for Electrochemical Double-Layer Capacitors

Abstract An active carbon (AC)/AC electrochemical capacitor, taking advantage of a high-concentrated lithium trifluoromethane sulfonate (LiTFS) or lithium bis(trifluoromethane sulfonyl) imide (LiTFSI) aqueous electrolyte, was demonstrated with an extended operating voltage of 2.5 V, which is the largest value till now for aqueous carbon-based capacitors. The AC electrode is entirely capacitive in these two electrolytes and the stable potential window of the single AC electrode can reach −1.2 to 1.2 V versus the saturated calomel electrode (SCE). The performance of the AC-based capacitor is evaluated in two- and three-electrode cells using a combination of electrochemical impedance (EIS), cyclic voltammetry (CV), galvanostatic discharge-charge, and self-discharge (SD, i.e., leakage current) measurements. At 0.5-mA cm−2 charge-discharge rate, the AC/AC capacitor presents 5.5 wh kg−1 and 4.5 wh kg−1 energy density for 20 m LiTFS and LiTFSI electrolyte, respectively. The results suggest that a thorough utilization of such lithium salt aqueous electrolytes with widening electrochemical stable potential window will no doubt lead to further development of electrochemical capacitors toward superior performance.

Nanothin film conductivity measurements reveal interfacial influence on ion transport in polymer electrolytes

Nanoscale interfacial zone limits ion transport properties of polymer electrolytes.

Low-Polarization Lithium-Oxygen Battery Using [DEME][TFSI] Ionic Liquid Electrolyte.

The room-temperature molten salt mixture of N,N-diethyl-N-(2-methoxyethyl)-N-methylammonium bis(trifluoromethanesulfonyl) imide ([DEME][TFSI]) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt is herein reported as electrolyte for application in Li-O2 batteries. The [DEME][TFSI]-LiTFSI solution is studied in terms of ionic conductivity, viscosity, electrochemical stability, and compatibility with lithium metal at 30 °C, 40 °C, and 60 °C. The electrolyte shows suitable properties for application in Li-O2 battery, allowing a reversible, low-polarization discharge-charge performance with a capacity of about 13 Ah g-1carbon in the positive electrode and coulombic efficiency approaching 100 %. The reversibility of the oxygen reduction reaction (ORR)/oxygen evolution reaction (OER) is demonstrated by ex situ XRD and SEM studies. Furthermore, the study of the cycling behavior of the Li-O2 cell using the [DEME][TFSI]-LiTFSI electrolyte at increasing temperatures (from 30 to 60 °C) evidences enhanced energy efficiency together with morphology changes of the deposited species at the working electrode. In addition, the use of carbon-coated Zn0.9 Fe0.1 O (TMO-C) lithium-conversion anode in an ionic-liquid-based Li-ion/oxygen configuration is preliminarily demonstrated.

Effect of propylene carbonate-Li+ solvation structures on graphite exfoliation and its application in Li-ion batteries

The effect of propylene carbonate (PC)-Li+ solvation structures on graphite exfoliation was investigated over a range of concentrations of PC-based electrolytes. At low concentrations of lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) in PC (1.3M and 2.1M), the graphite anode was exfoliated. However, the graphite exfoliation could be effectively suppressed when the concentrations of dissolved LiTFSI were increased to 2.5M and 3.3M. The results of spectroscopic analyses and density functional theory (DFT) calculations revealed that electrochemical exfoliation of the graphite anode is closely associated with a special spatial configuration of Li+-(PC)n (1≤n≤4) solvation structures at various Li-salt concentrations and corresponding solid electrolyte interface (SEI) film formation mechanisms. When the concentration of LiTFSI increased from 1.3 to 3.3M, the spatial configuration of Li+-(PC)n (1≤n≤4) solvation gradually changed from a tetrahedron (occupied space of 10.19Å) to planar (occupied space of 3.05Å), which reduced the structure change for co-intercalation into the graphite interlayers of Li+-(PC)n (1≤n≤4) solvates. Meanwhile, the affinity between Li+-(PC)n (1≤n≤4) solvation cations and TFSI− anions was increased, leading to the significant contribution of TFSI− anions to SEI formation on the surface of graphite. Additionally, Al corrosion was not of concern in concentrated LiTFSI electrolyte. The 3.3M LiTFSI/PC concentrated electrolyte exhibits promising electrochemical performance in graphite||LiNi1/3Co1/3Mn1/3O2 full cells.

Polymeric ionic liquid-plastic crystal composite electrolytes for lithium ion batteries

In this work, composite polymer electrolytes (CPEs), that is, 80%[(1−x)PIL–(x)SN]–20%LiTFSI, are successfully prepared by using a pyrrolidinium-based polymeric ionic liquid (P(DADMA)TFSI) as a polymer host, succinonitrile (SN) as a plastic crystal, and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) as a lithium salt. XRD and DSC measurements confirm that the as-obtained CPEs have amorphous structures. The 80%[50%PIL–50%SN]–20%LiTFSI (50% SN) electrolyte reveals a high room temperature ionic conductivity of 5.74 × 10−4 S cm−1, a wide electrochemical window of 5.5 V, as well as good mechanical strength with a Young's modulus of 4.9 MPa. Li/LiFePO4 cells assembled with the 50% SN electrolyte at 0.1C rate can deliver a discharge capacity of about 150 mAh g−1 at 25 °C, with excellent capacity retention. Furthermore, such cells are able to achieve stable discharge capacities of 131.8 and 121.2 mAh g−1 at 0.5C and 1.0C rate, respectively. The impressive findings demonstrate that the electrolyte system prepared in this work has great potential for application in lithium ion batteries.

Electrochemical Investigation of the Solid Electrolyte Interface on Lithium Electrodes Cycled with a Glyme-Litfsi Electrolyte

Due to their high thermal stability, good compatibility with lithium, and nonflammability, room temperature ionic liquids (RTILs) based electrolytes have attracted considerable attention for application in lithium-sulfur batteries and lithium-air batteries [1, 2]. However, two limiting factors have so far been the low Li+ concentration and low Li+ transference number [3]. A quasi-ionic liquid consisting of glymes and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), in which the lithium cation is the only cation species, has recently been proposed as a way to address these issues [4]. This complex exhibits similar advantages as RTILs but has a higher ionicity and a higher lithium transference number and has shown to contribute to good performance in lithium-sulfur batteries [5, 6].In this contribution we report on the formation of the solid electrolyte interphase (SEI) on lithium electrodes in glyme (tetra ethylene glycol dimethyl ether, G4) - LiTFSI electrolytes with different molar ratios (G4:LiTFSI, 8:1 to 1:1). As the molar ratio (G4:LiTFSI) decreases the cycling efficiency, in terms of lithium deposition and dissolution on Cu disks, increases from 20% to around 95% for the 1:1 complex. The cycling behavior and impedance measurements in symmetric cells show that the electrolytes with various molar ratio of G4 to LiTFSI result in SEI films with different properties. Furthermore, the SEI film formed in the electrolyte with the equimolar ratio is relatively stable and our results indicate that the equimolar complex of G4 and LiTFSI has an improved compatibility with the lithium electrode. From X-ray photoelectron spectroscopy (XPS), with argon-ion sputtering for depth profiling, we find that the relative amount of fluorinated species, such as LiF and C-F, in the films increases with increasing molar ratio of G4 to LiTFSI. This suggests that the breakdown of TFSI anions is more likely to happen in electrolytes with higher glyme concentration. These results are of importance when considering the implementation of G4-LiTFSI complexes as electrolytes in Li-metal type batteries.Fig. 1. Cycling efficiency (EFF) of the lithium deposition and dissolution on Cu disk in [Li(G4)x][TFSI] (x = 1, 2, 4, and 8).AcknowledgementsThe support provided by China Scholarship Council (CSC) during a visit of Shizhao Xiong to Chalmers is acknowledged. We are grateful to the support of the Areas of Advance Energy, Materials Science, and Transport at Chalmers.

Concentrated electrolytes: decrypting electrolyte properties and reassessing Al corrosion mechanisms

Highly concentrated electrolytes containing carbonate solvents with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) have been investigated to determine the influence of eliminating bulk solvent (i.e., uncoordinated to a Li+ cation) on electrolyte properties. The phase behavior of ethylene carbonate (EC)–LiTFSI mixtures indicates that two crystalline solvates form—(EC)3:LiTFSI and (EC)1:LiTFSI. Crystal structures for these were determined to obtain insight into the ion and solvent coordination. Between these compositions, however, a crystallinity gap exists. A Raman spectroscopic analysis of the EC solvent bands for the 3–1 and 2–1 EC–LiTFSI liquid electrolytes indicates that ∼86 and 95%, respectively, of the solvent is coordinated to the Li+ cations. This extensive coordination results in significantly improved anodic oxidation and thermal stabilities as compared with more dilute (i.e., 1 M) electrolytes. Further, while dilute EC–LiTFSI electrolytes extensively corrode the Al current collector at high potential, the concentrated electrolytes do not. A new mechanism for electrolyte corrosion of Al in Li-ion batteries is proposed to explain this. Although the ionic conductivity of concentrated EC–LiTFSI electrolytes is somewhat low relative to the current state-of-the-art electrolyte formulations used in commercial Li-ion batteries, using an EC–diethyl carbonate (DEC) mixed solvent instead of pure EC markedly improves the conductivity.

Improved electrochemical performance of LiMO2 (M=Mn, Ni, Co)–Li2MnO3 cathode materials in ionic liquid-based electrolyte

Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in N-butyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide (PYR14FSI) (1:9 in molar ratio) is successfully tested as electrolyte for the high voltage LiMO2–Li2MnO3 (cathode)/lithium (anode) cells at elevated temperature (40 °C). Compared to conventional electrolytes, such as 1 M LiPF6 solution in the mixed solvent of ethylene and dimethyl carbonate (EC:DMC = 1:1), the use of PYR14FSI–LiTFSI electrolyte results in a net improvement of LiMO2–Li2MnO3 cycling stability while granting comparable initial capacity. In addition, the ionic conductivity of the ionic liquid-based electrolyte at 40 °C is high enough to sustain the excellent rate capability of this cathode material. Li/LiMO2–Li2MnO3 cells delivered initial capacity exceeding 200 mA h g−1 at high current rate (2 C) while retaining 94% of the initial capacity after 100 cycles. Differential capacity versus potential analysis and post-mortem characterization by scanning electron microscope, X-ray diffraction and were carried out to explain the improved performance of LiMO2–Li2MnO3 in the IL-based electrolyte.

Distinct Difference in Ionic Transport Behavior in Polymer Electrolytes Depending on the Matrix Polymers and Incorporated Salts

Two different electrolyte salts, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and a room temperature ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMITFSI), were incorporated into network polymers to obtain ion-conductive polymer electrolytes. Network polymers of poly(ethylene oxide-co-propylene oxide) (P(EO/PO)) and poly(methyl methacrylate) (PMMA) were chosen as matrixes for LiTFSI and EMITFSI, respectively. Both of the polymer electrolytes were single-phase materials and were completely amorphous. Ionic conductivity of the polymer electrolytes was measured over a wide temperature range, with the lowest temperatures close to or below the glass transition temperatures (Tg). The Arrhenius plots of the conductivity for both of the systems exhibited positively curved profiles and could be well fit to the Vogel-Tamman-Fulcher (VTF) equation. The conductivity of the PMMA/EMITFSI electrolytes was higher at most by 3 orders of magnitude than that of the LiTFSI/P(EO/ PO) electrolytes at ambient temperature. When the ideal glass transition temperature, T0 (one of the VTF fitting parameters), was compared with the Tg, a difference in the ionic conduction was apparent in these systems. In the P(EO/PO)/LiTFSI electrolytes, the T0 and Tg increased in parallel with salt concentration and the T0 was lower than the Tg by ca. 50 degrees C. On the contrary, the difference between the T0 and the Tg increased with increasing content of PMMA in the PMMA/EMITFSI electrolytes, with the observed difference in the concentration range studied reaching up to ca. 100 degrees C. The conductivity at the Tg, sigma(Tg), for the LiTFSI/P(EO/PO) electrolytes was on the order of 10(-14-)10(-13) S cm(-1) and increased with increasing salt concentration, whereas that for the PMMA/EMITFSI polymer electrolytes reached 10(-7) S cm(-1) when the concentration of PMMA was high. The ion transport mechanism was discussed in terms of the concepts of coupling/decoupling and strong/fragile for the two different polymer electrolytes.